Power system with low voltage ride-through capability

ABSTRACT

A wind powered turbine with low voltage ride-through capability. An inverter is connected to the output of a turbine generator. The generator output is conditioned by the inverter resulting in an output voltage and current at a frequency and phase angle appropriate for transmission to a three-phase utility grid. A frequency and phase angle sensor is connected to the utility grid operative during a fault on the grid. A control system is connected to the sensor and to the inverter. The control system output is a current command signal enabling the inverter to put out a current waveform, which is of the same phase and frequency as detected by the sensor. The control system synthesizes current waveform templates for all three-phases based on a sensed voltage on one phase and transmits currents to all three-phases of the electrical system based on the synthesized current waveforms.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of co-pending U.S. patentapplication Ser. No. 10/981,364, which was filed on Nov. 11, 2004, andwhich itself was a Continuation In Part of co-pending U.S. patentapplication Ser. No. 10/773,86 which was filed on Feb. 4, 2004, now U.S.Pat. No. 7,042,110, and which claimed priority under 35 U.S.C. § 119(e)to U.S. Provisional Patent Application No. 60/18,899 Variable Speed WindTurbine Technology which was filed on May 7, 2003 and which isincorporated herein by reference.

This application is related to U.S. Pat. No. 6,304,002, U.S. Pat. No.6,731,017, U.S. Pat. No. 6,653,744, and U.S. patent application Ser. No.10/426,287 Kevin L. Cousineau: Distributed Static VAR Compensation(DSVC) System For Wind And Water Turbine Applications” filed Apr. 30,2003, and U.S. patent application Ser. No. 10/449,342 of Amir S. Mikhailand Edwin C. Hahlbeck entitled “Improved Distributed Power Train (DGD)With Multiple Power Paths” filed May 31, 2003, all of which are assignedto Clipper Windpower Technology, Inc. and are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to generators with current regulated inverters,including wind and water current turbines, and more particularly tovariable speed wind turbines employing multi-phase generators using fullpower conversion systems with utility fault ride through capability.

2. Description of the Prior Art

The application of wind-powered generating systems in the past has beenon a small scale when compared to the total generating capacity of theelectrical grid. A term which is often used to describe the relativequantity of the wind generated power is “penetration”. Penetration isthe ratio of wind generated power to the total available generatedpower. Even in those states where wind generated power is highest, thepenetration levels are under, or about one percent. While this is arelatively small amount of power, and the rules that govern theoperation of the turbines reflect this small penetration, it is clearthat the operating rules are changing. This is in anticipation ofsubstantially higher penetration levels into the future. One operatingprinciple that is being revised is how a wind turbine reacts to a faulton the utility grid transmission (or sub-transmission) system to whichthe wind turbine is interconnected. A fault is defined as a utilitydisturbance that results in voltage on the utility transmission systemdecreasing by a significant amount for a short duration (typically lessthan 500 milliseconds). Faults can be caused by at least one phaseconductor being inadvertently connected to ground (a ground fault), orthe inadvertent connection, or short circuiting of multiple phaseconductors. These types of faults occur during lightning and windstorms, or when a transmission line is involved in a vehicle accident,as examples. A significant reduction in voltage can also occur when alarge change in electrical load or electrical generation occurs nearbyin the utility transmission system. Examples of this type of event couldinclude sudden disconnection of a large power plant, or suddenconnection of a large load such as a steel mill. This type of a voltagereduction event is not typically referred to as a fault in utilityparlance, although for the purposes of this specification the term“fault” is intended to cover such voltage reduction events. The term“fault” as used herein, is intended to cover any event on the utilitysystem that creates a momentary reduction or increase in voltage on oneor more phases. In the past, under these inadvertent fault and largepower disturbance circumstances, it has been acceptable and desirablefor a wind turbine to trip off line whenever the voltage reductionoccurs. Operating in this way has no real detrimental effect on thesupply of electricity when penetration is low. This operating rule isunder revision however, and it is now desirable for a wind turbine toremain on line and ride through such a low voltage condition. This newoperation is similar to the requirements applied to traditionalgenerating sources such as fossil fueled synchronous generator plants.The reason for this requirement is straight forward; if wind generatedpower is at a high level of penetration, and a momentary fault occurs,the dropping of the significant amount of wind generated power (asrequired under the old operating rules) can cause much more seriousstability problems, such as frequency swings, or large system wideinstabilities of generation systems. These are very extensive faultconditions and can lead to the disruption of power to large regions,effecting large numbers of utility customers. Using variable speed windturbines to generate electrical power has many advantages that includehigher blade efficiency than constant speed wind turbines, control ofreactive power-VARs and power factor, and mitigation of mechanicalturbine drivetrain loads. The low voltage ride through requirementdescribed above, often referred to as utility fault ride through, isalso more easily addressed using certain variable speed wind turbinetechnology as will be disclosed herein. In considering variable speedwind turbines, it is important to examine two classes of powerconverters which are used and which could be used for the utility ridethrough function.

One prior art variable speed wind turbine uses a total conversion systemto completely rectify the entire power output of the wind turbine. Thatis, the wind turbine, operating at a variable frequency and variablevoltage, converts this power into a fixed frequency and voltage thatmatches that of the grid. An example of this type of system is disclosedin U.S. Pat. No. 5,083,039 (incorporated herein by reference) whichcomprises a turbine rotor that drives a pair of AC squirrel cageinduction generators with two respective power converters that convertthe generator output to a fixed DC voltage level. The DC bus of thissystem is then coupled to the utility inverter and power is inverted atfixed frequency and supplied back to the utility. The generator controlsystem in U.S. Pat. No. 5,083,039 uses field orientation principles tocontrol torque and uses real and reactive power control methods tocontrol the utility inverter. While generation in this turbine requiresonly unidirectional power flow, a bidirectional converter is inherentlyrequired as the induction generators need to be excited from the DC bus.The DC bus in this system is controlled from the utility inverterportion of the conversion system and control of the DC bus is difficultwhen the utility voltage falls substantially.

A second example of a total conversion system is that disclosed inabove-identified U.S. patent application Ser. No. 10/773,86. This systemutilizes synchronous generators together with a passive rectifier andactive utility inverter to convert generator variable frequency andvoltage to utility compatible frequency and voltage. This system isinherently unidirectional in its ability to pass power from thegenerator to the grid. An advantage of this system is that the DC bus iscontrolled from the generator side of the power conversion system andbus control is straight forward during the periods of low utilityvoltages.

U.S. Pat. Nos. 6,137,187 and 6,420,795 (both incorporated herein byreference) describe a partial conversion, variable speed system for usein wind turbines. The system comprises a wound rotor inductiongenerator, a torque controller and a proportional, integral derivative(PID) pitch controller. The torque controller controls generator torqueusing field-oriented control and the PID controller performs pitchregulation based on generator rotor speed. Like the U.S. Pat. No.5,083,039 patent, power flow is bi-directional within the rotor of thegenerator and an active rectifier is used for the conversion process.The converter used in this system is rated at only a portion of thetotal turbine rating, with the rating depending on the maximum generatorslip desired in the turbine design. The converter controls the currentand frequency in the rotor circuit only with a direct electricalconnection between the generator stator and the utility. In addition tothe converter controlling torque in this system, the converter iscapable of controlling system reactive power or power factor. This isaccomplished by under/over exciting the generator rotor circuit alongits magnetization axis. The converter is connected in parallel to thestator/grid connection and only handles rotor power input and output.This system is difficult to control in the event of a sudden drop inutility voltage. This is because the rotor converter DC bus iscontrolled from the utility side converter just as in the U.S. Pat. No.5,083,039 patent and because the generator stator is directly connectedto the utility. The direct stator connection creates problems in that noconverter is between the stator and utility and transient currents andtorques are generated which are not subject to control by an interveningconverter.

U.S. Pat. No. 7,042,110 (incorporated herein by reference) describes asystem for regulating a wind turbine connected at the utilitydistribution level based on the voltage of the system. U.S. Pat. No.7,042,110 stands in contradistinction to the fact that most windgeneration in the United States is connected at the sub-transmissionlevel. Moreover, the method described does not address the sudden, deepdrop of utility voltage.

It is desirable to provide a variable speed wind or water currentturbine, which has the ability to continue inverter control during autility fault, such as a sudden, deep drop of utility voltage.

It is also desirable to provide a ride-through capability for a wind orwater current turbine system, in which the generator is completelydecoupled from a utility grid and its disturbances.

SUMMARY OF THE INVENTION

Briefly, the invention is an apparatus and method of controlling agenerator in which a measurement of voltage frequency and phase angle onone phase is made, a synthesis of current waveform templates for allphases is made based on the voltage measurement from the one phase, andbased upon the current waveform, electrical current is delivered to autility grid during a fault condition at a level that is substantiallythe same as pre-fault conditions.

The invention has the advantage that it has the ability to continueinverter control in a variable-speed wind-turbine system during autility fault.

The invention has the advantage that it provides a method forsynthesizing balanced three-phase current reference waveform templatesunder conditions where the utility is fully functioning, but also whenthere are one or more faults present on the utility transmission andcollection system.

The invention has the advantage that it relies on only one phase of thethree-phase system to be operational, and that phase need only beoperational down to approximately 5% of rated voltage.

This invention has the advantage that the system only requires a smallvoltage level for synchronizing, approximately 5% voltage, on the singlesensed phase of the three-phase system, the current references andtherefore the inverter currents remain unaffected by a wide range offaults. Ground fault conditions, or phase-to-phase faults on the twonon-sensed phases, have little or no affect on the references andutility currents. Ground faults on the single sensed phase, at thetransmission or collection system level will typically produce more than5% voltage given typical wind farm system impedances.

This invention has the advantage that the generator is completelydecoupled from the grid (and its disturbances) by the total converter.The partial converter system, by contrast, is not completely decoupled,as the stator is directly connected to the utility grid, and griddisturbances cause large transients which cannot be buffered ordecoupled by the converter.

This invention has the advantage that in the system provides utilitydisturbance and fault ride through ability via a robust currentreference synthesis function and simplified generator torque commandapproach.

This invention has the advantage that in the system providessynthesizing of the three-phase current references from a single sensedphase.

This invention has the advantage that in the system provides operationof the three-phase synthesizing function down to very low voltage,approximately 5% line voltage, during a fault condition on the sensedphase.

This invention has the advantage that in the system providesdistortion-free, current templates via the use of sinusoidal,three-phase, balanced lookup tables or computed trigonometric sinefunction.

The invention has the further advantage that in order for this system tooperate there only needs to be a detectable frequency signal on thepower line at the output of the inverter. Since frequency is detectedeven during a utility fault condition, the inverter continues to injectcurrent into the line in balanced three-phase fashion with a near puresinusoidal shape at the detected frequency with the appropriate phaseangle for all three-phases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electrical schematic of an array of wind turbinesemploying current regulated pulse-width-modulated (PWM) invertersconnected to a wind farm collection system and utility sub-transmissionsystem in which the present invention is embodied;

FIG. 2 shows an electrical schematic of a single current regulated PWMinverter and variable speed wind turbine according to an embodiment ofthe present invention;

FIG. 3 shows a time series of phase to ground and phase to phase voltageon three-phases of the utility system before, during, and after autility fault;

FIG. 4 shows an expanded time series of current injected into theutility system during the time period when an instantaneous faultinitiates;

FIG. 5 shows a time series of current injected into the utility systemduring the time period when a fault instantaneously clears; and,

FIG. 6 shows one embodiment of an inverter control circuit according tothe present invention utilizing a phase-locked loop.

DETAILED DESCRIPTION OF THE INVENTION

Refer to FIG. 1, which shows a collection of generators with currentregulated inverter systems in the form of a wind or water current energyfarm 1. Individual wind turbines 3 are connected to a wind farmcollection system 5. The energy farm collection system 5 may interfacewith a utility grid distribution, sub-transmission, or transmissionsystem 7 via a substation transformer 9. The energy farm collectionsystem 5 may isolate wind turbine groups 11 using sectionalizing devices13. The sectionalizing devices, often circuit breakers or fuses, isolatea group of turbines 11 in case of an electrical fault within the turbinegroup 11, thus allowing the rest of the windfarm 1 to continueoperating. Groups of turbines (such as 11) are connected in common tothe energy farm collection system 5 for interfacing with a utility grid7.

The energy farm 1 is made up of fluid-flow turbines 3, shown in detailin FIG. 2. Referring now to FIG. 2, each fluid-flow turbine has a rotor15. An output of the rotor is rotational power. A generator 17 isconnected to the output of the rotor, an output of the generator beingAC electrical power. A rectifier 19 converts the AC power to DC, whichis connected to DC link 21. An inverter 23 is connected to the output ofthe DC link 21, at least a portion of the electrical power output of thegenerator 17 being conditioned by the inverter resulting in an inverteroutput voltage and current at a frequency and phase angle appropriatefor transmission to the utility grid 7 shown in FIG. 1. Each fluid-flowturbine has a control system 24 having a control system input and acontrol system output connected to the inverter 23.

The energy farm shown in FIG. 1 has a collection system 5 connected tothe utility grid. A group of the fluid-flow turbines 11 have theirinverter outputs connected to the collection system 5. A frequency andphase angle sensor is connected to the utility grid 7 at an appropriatepoint to operate during a fault on the grid. Each turbine 3 has itscontrol system input connected to the sensor. Each control system (24 inFIG. 2) produces an output that is a current command signal enabling theinverter to which it is connected to put out a current waveform which isof the same phase and frequency as detected by the sensor. Instead of acommon sensor, each turbine may have its own sensor 30, as shown in FIG.2.

In addition to the sectionalizing devices 13, the energy farm typicallyalso includes additional isolation and protection devices at thesubstation 9 and also within the controller of each individual windturbine 3. Such additional protection devices would typically includeover and under voltage and over and under frequency trip mechanisms.These trip mechanisms are coordinated with each other and with thesubstation to provide a desired protection scheme.

Refer to FIG. 2 in which components of a single wind turbine 3 of FIG. 1are shown schematically. A rotor 15 converts energy in fluid flow, suchas wind or water current, into rotational kinetic energy. A generator 17converts the rotational kinetic energy into variable frequency ACelectrical power. A rectifier 19 converts the AC power to DC. A DC link21 has some DC energy storage capability for stabilizing smalltransients. A current regulated inverter 23 converts the DC power to ACpower at utility line frequency. An inverter control circuit 24incorporates many functions of turbine control. A protection device 25,such as a circuit breaker and/or fuse, is provided for isolating theturbine 3 in case of a fault. A pad-mount transformer 27 changes thevoltage of power produced by the turbine to the voltage of the energyfarm collection system 5 shown in FIG. 1. The wind turbine usuallyproduces power at a low voltage such as 575 VAC or 690 VAC and thecollection system is typically at a higher voltage such as 34.5 kV.

The wind turbine 3 and collection system 5 are shown as operating withthree-phase power. The present invention could include the use ofsingle-phase power or multi-phase power with any number of phases.Design of the rotor 15 shown in FIG. 2 is within the skill of one ofordinary skill in the art and would be accomplished using the techniquesdescribed in Wind Energy Handbook written by Burton, Sharpe, Jenkins,and Bossanyi and published by John Wiley & Sons in 2001, Wind PowerPlants: Fundamentals, Design, Construction and Operation written byGasch and Twele and published by James & James in 2002, Wind TurbineEngineering Design, written by Eggleston and Stoddard and published byVan Nostrand Reinhold in 1987, Windturbines, written by Hau andpublished by Springer in 2000, Wind Turbine Technology, edited by Speraand published by ASME Press in 1994, and Wind Energy Conversion Systems,written by Freris and published by Prentice Hall in 1990, all of whichare incorporated herein by reference. Information about transformerdesign, grounding, power quality, and other aspects of energy farmintegration with the utility grid can be found in Grid Integration ofWind Energy Conversion Systems, written by Heier and published by JohnWiley & Sons, Inc, 2002, ISBN: 0-471-97143-X which is incorporatedherein by reference.

The inverter control circuit 24 can be relatively simple or very complexincorporating many functions of turbine control. The inverter controlcircuit may be an independent circuit simply for the functions relatedto the technique of the present invention or may simply be a part of theinverter or some other component of the wind turbine system or aspectsof the control circuit 24 spread out among components. The invertercontrol circuit 24, shown in FIG. 2, is less a separate physicalcomponent of the wind turbine but rather is shown to illustrate thetechnique of the present invention. The inverter control circuit 24contains those elements normally used in the regulation of AC linecurrents as described for example in Ned Mohan, Tore M. Undeland,William P. Robbins Power Electronics: Converters, Applications, andDesign Publisher: John Wiley & Sons; 3rd edition (October 2002) ISBN:0471226939 and W. Leonhard, Control of Electrical Drives,Springer-Verlag, 1985, both of which are incorporated herein byreference.

The inverter control circuit 24 senses a voltage signal 30 from a singlephase of the low voltage side of the pad-mount transformer 27. Thetechnique of the present invention will work by sensing voltage fromonly one phase but it is conceived that the inverter control circuit 24could sense all three-phases and in the case of a fault condition chooseto track the strongest of the three or track all three independently.The inverter control circuit 24 may utilize only frequency and phaseinformation from the received signal 30. The amplitude of the voltagesignal is relatively unimportant. Frequency and phase can be detectedeven if the voltage on the line is significantly reduced. In fact, evenif voltage is zero at a fault point at a distant location on the utilitycollection, distribution, sub-transmission, or transmission system,impendence between the generator and the fault will still create avoltage waveform as long as current is supplied.

FIG. 3 shows the phase to ground and phase to phase voltage waveforms atthe wind turbine connection location before 18, during 20 and after 22 asimulated single phase to ground fault (the most common type of fault).It can be seen that even the faulted phase still has a detectablefrequency and phase. Other types of transmission faults including, phaseto phase, and three-phase symmetrical fault would show similarwaveforms, all of which would have detectable phase and frequency ifcurrent is supplied from the generator. Once the frequency and phase ofa voltage waveform have been determined, the inverter control circuit 24then generates a current command signal 32 (within the broken lines)that instructs the inverter 23 to put out a current waveform template toprotection device 25, which template is of the same phase and frequency.In a balanced three-phase system this would consist of one phase beingat 0 degrees, one phase being shifted 120 degrees and a third phasebeing shifted at 240 degrees. The current waveform is unlike thedetected voltage waveform 30 in that it is nearly perfectly sinusoidalin shape (which the voltage may not be leading up to and during a faultcondition) and its magnitude is not dependent on line voltage magnitude.The protection device 25 is connected to pad-mount transformer 27, whichchanges the voltage of power produced to the voltage of the energy farmcollection system 5.

The current command signals 32 may be generated digitally using look-uptables or using analog circuitry, or it may be a software routineexecuting a trigonometric sine function. For the wind turbine case beingdiscussed herein, the strategy is to leave the AC current command 32level constant during the fault. This is done because faults asdiscussed herein are short in duration and the impacts on the windturbine system are minimal. Also, when the utility returns to normalvalues, the wind turbine system picks up right where it left offprevious to the fault, in a seamless manner. Before 18, during 20, andafter 22, (a fault) the current regulated inverter applies the samecurrent supply to the utility system with only a minor disturbance inthe current.

FIG. 4 and FIG. 5 show simulated current waveforms from the generator atonset and termination of a single-phase ground fault condition,respectively. In this way a generator with a current regulated invertermay be caused to “ride through’ a brief fault condition without trippingoff-line or adding substantial disturbance to the collection andsub-transmission system at termination of the fault. One technique forachieving the above function of the inverter circuit controller 24 is byutilizing a phase-locked loop, a technique familiar to those skilled inthe art and described in F. M. Gardner, Phaselock Techniques (2ndEdition), Wiley (1979) or Roland E. Best, Phase-locked Loops,McGraw-Hill (1993), both of which are incorporated herein by reference.Other techniques than a phase-locked loop are certainly possible forachieving the desired effect and are intended to be within the scope ofthe present invention. The inverter control circuit 24 may be embodiedas a physical hardware component or it can be implemented in softwareusing a microprocessor.

FIG. 6 shows elements of an inverter control circuit 24 utilizing aphase-locked loop. Showing an inverter control circuit 24 in this mannerin no way limits the technique of the present invention to this specifictopology but rather illustrates one way in which the technique of thepresent invention may be implemented. The sensed line voltage 30 ismultiplied 33 by a corrective signal 34 of a phase-locked loop. Sincethe corrective signal 34 is ideally ninety degrees out of phase with theinput signal 30 and both signals are at 60 Hertz, the resultant signal36 is a sinusoidal signal of 120 Hz with a DC offset related to thephase difference between the input signal 30 and the corrective signal34. A phase difference of exactly ninety degrees produces zero DCoffset. The resultant signal 36 is then passed into a low pass filter 38which removes the 120 Hz component from the resultant signal leavingonly a DC signal 40. This DC signal 40 is passed through aproportional-integral (P-I) regulator 42 which is set to control thedynamics of the inverter control circuit 24 response. The P-I output 44is a DC signal which is added to an output 1 of a triangle wavegenerator 48. The triangle wave command signal regardless of the voltageon the utility line. Techniques for producing an inverter capable ofsuch current insertion are well known to those skilled in the art andcan be found in Ned Mohan, Tore M. Undeland, William P. Robbins PowerElectronics: Converters, Applications, and Design Publisher: John Wiley& Sons; 3rd edition (October 2002) ISBN: 0471226939 and W. Leonhard,Control of Electrical Drives, Springer-Verlag, 1985.

To illustrate the present invention, its operation is described as itwould function on a wind turbine 3 similar to that disclosed in U.S.patent application Ser. No. 10/773,86. In the case of a fault, theinverter control circuit 24 commands substantially the same current forthe duration of the fault condition as existed immediately prior to thefault. The control technique used on the above mentioned wind turbine 3controls current output based on desired torque at the rotor 15. Desiredtorque control has a time constant on the order of seconds compared tothe milliseconds of fault duration. During a fault, shown as timeduration 20, voltage is lower than normal on at least one of the phases(see FIG. 3). Thus with equal current flowing as in the pre-fault periodof time (designated as the 18 condition) there is less power transmittedto the grid during the period of the fault 20. In the case of the abovementioned turbine 3 this causes the voltage on the DC link 21 to riseslightly which will decrease the torque that the generator 17 applies tothe rotor 15 causing the rotor 15 to accelerate slightly. The amount ofenergy which needs to be absorbed over 150 ms or even 500 ms is smallenough that it is well within the allowed speed range of the rotor 15.After the fault has passed 22, the wind turbine 3 regulates itselfnormally, dissipating the extra energy into the utility or throughpitching of the rotor blades. In fact the amount of energy whichgenerator 48 produces a steady sixty hertz triangle wave. The P-I outputsignal 44 and the triangle wave signal are summed 50. The summed signal52 is then passed through a cosine function 54 which is the correctivesignal 34. The P-I output signal 44 is scaled such that by adding it tothe triangle wave 1 the corrective signal 34 is phase shifted to bringit closer to ninety degrees out of phase with the input signal 30. Thesummed signal 52 is connected to a number of parallel phase shiftcircuits 61, 63, 65, wherein the summed signal undergoes a phase shiftfunction of −240° 61, a phase shift function of −120° 63, and a phaseshift function of −0° 65 in parallel fashion. A number of trigonometricsine function circuits 56 are connected to the number of parallel phaseshift circuits producing a fixed amplitude reference sinusoidal wave 58at sixty Hertz (or other frequency as appropriate for the utilityfrequency) substantially in phase with the utility. In this way, abalanced three-phase set of current-reference signals of unity amplitude58 are generated. The reference sinusoidal waveform set 58 is thenscaled by a number of scaling multiplier circuits 59 by multiplying a DCvalue 60 that corresponds to the desired AC output current levelproducing a scaled sinusoidal signal which is the current command signal32. This DC value is set by a turbine controller to set the AC currentlevel. This current level is roughly proportional to the generatortorque level and is based on a number of inputs. The current commandsignal 32 will set the inverter output current substantially in phasewith the voltage of the three-phases from the utility. The magnitude ofthe scaling signal 60 is not directly determined by the input voltagesignal 30 and thus the amount of current commanded of the inverter 23does not change substantially with changes of the input voltage signal30. The inverter 23 will inject current into the utility line withmagnitude, frequency, and phase according to the current must beabsorbed in order to allow a generator to operate through a transientfault is small enough that the energy could simply be stored as avoltage rise in the DC link 21 (or equivalent in other types ofgenerators). Energy storage of this magnitude (tens to hundreds ofwatt-hours for a 1.5 MW generator depending on type and duration offault) is readily available, super capacitors and high instantaneousamperage batteries being examples of such energy storage devices. Theabove example is for the sole purpose of illustrating the technique ofthe present invention in a specific application but in no way limits thescope of the invention.

The ability to ride though such faults is imperative for such types ofgeneration to encompass a significant percentage of generation on adistribution system. Most loads on the distribution system are expectingthe same power availability after a fault 22 as before 18 so if a largeportion of the generation on a distribution system goes off-line ordelivers poor power quality due to such a fault then the reliability ofthe utility system is compromised. Currently this is significant to thewind generation industry which until now has been a small part of thenational electric supply. Wind power generation's rapid growth hascaused it to become a significant source of power in some regions and isprojected to reach a significant percentage of the nation's electricalsupply in the foreseeable future. Thus, providing wind generators withthe ability to ride through a grid fault condition (such a requirementis in place for most other major generation sources) is a loomingnecessity. This same necessity will apply to other forms of currentsource generation as those technologies gain significant penetrationlevels.

The present invention is shown and described in a number of differentembodiments. There are other embodiments of this invention beyond thosespecifically described. These other embodiments although not explicitlydescribed herein are implicit from or will be understood from thedescribed embodiments by one skilled in the art.

The present invention involves a generator with a current regulatedinverter system interconnected with an electrical conducting system. Inthis specification the generator with a current regulated inverter isdescribed as a full conversion wind turbine system consistent with U.S.Pat. No. 5,083,039. This generation system rectifies the full output ofthe wind generator to produce DC electricity, which is then convertedback to AC at the utility frequency and phase by a current regulatedpulse-width-modulated (PWM) inverter. Those skilled in the art willunderstand that other generators with a current regulated inverter mayemploy the technique of the present invention including other topologiesof wind turbines, water current turbines, fuel cell systems,photo-voltaic systems, diesel generators, and other power generationsources. Furthermore, the present invention may also be utilized with avariable speed turbine that uses partial conversion of the generatoroutput as described in U.S. Pat. Nos. 6,137,187 and 6,420,795. Theinvention may be utilized with a wind turbine that includes either asynchronous generator or an induction generator.

In this specification the electrical conducting system is described asan electrical utility grid with the generator and current regulatedinverter connected to a collection system and further to thesub-transmission level collectively with similar generators though asubstation transformer. These specifics are only for illustrativepurposes, as this is a typical way that the energy from wind turbines isinterconnected with the utility system. The technique of the presentinvention works for a single generator as well as for a collectivegroup. The present invention may also be used in connection with theutility distribution level as well as the very high voltage transmissionlevel of a utilities distribution system. Furthermore this technique maybe employed in a stand-alone application or in a small isolated villagepower system.

The above examples of alternate current source generators and electricalconduction systems are intended to demonstrate the non-exclusive natureof the technique of the present invention and are in no way limiting.Those skilled in the art will realize that although the invention isdescribed as a total conversion system, it can also be applied to therotor converter portion of partial conversion systems. In the lattercase, the ability to ride through a utility disturbance is stillhindered by the direct connection of the stator to the utility, whichcannot be buffered by the converter system. More generally the inventiondescribed herein relates to a technique for utility fault ride thoughfor any generator with a current regulated inverter system.

Those skilled in the art will also realize that by looking at only onephase of the three-phase system to determine frequency and electricalangle, a balanced three-phase current template is constructed and usedfor purpose of controlling the wind turbine inverter current. By lookingat only one phase of the three-phase system, the other two phases aresynthesized at −120 and −240 electrical degrees to form a balancedthree-phase system of reference currents. The inverter used is typicallya current regulated, pulse-width-modulated inverter, often referred toas a CRPWM type of inverter. This inverter system has the ability toinstantaneously regulate utility currents by following a set ofreference currents as generated by the invention herein described. Thoseskilled in the art will understand that other types of current regulatedinverters could also be used such as current PWM current sourceinverters and multi-level inverters.

1-27. (canceled)
 28. A power system with low-voltage ride-throughcapability comprising: a primary source of power; a generator thatconverts said primary source of power into electrical power; an inverterthat conditions at least a portion of said electrical power to voltageand current at a frequency and phase angle appropriate for transmissionto an AC utility grid; a sensor that detects the frequency and phaseangle of the voltage on one phase of said utility grid during a fault onsaid grid; and, a control system for said inverter that controls saidinverter to provide current to said grid at a frequency and phase anglethat are based on said detected voltage frequency and phase angle. 29.The power system of claim 28 wherein said generator is a synchronousgenerator and further comprising a rectifier for converting AC powerfrom said generator into DC power.
 30. The power system of claim 28wherein said power system is a wind turbine and wherein said primarysource of power is a power in the wind and further comprising a windturbine rotor for converting power in the wind into rotational power.31. The power system of claim 28 wherein said utility grid is amulti-phase system and wherein said sensor detects the frequency andphase angle of the voltage on one phase of said multi-phase utility gridduring a fault on said grid.
 32. The power system of claim 28 whereinsaid sensor monitors several phases of a multi-phase utility grid andselects one phase on which to detect the frequency and phase angle ofthe voltage.
 33. The power system of claim 28 wherein said controlsystem controls said inverter to provide current to said grid atsubstantially the same magnitude as provided immediately before saidfault on said grid.
 34. The power system of claim 28 wherein saidcontrol system is implemented in hardware as an analog circuit.
 35. Thepower system of claim 28 wherein said control system is a phase-lockedloop.
 36. The power system of claim 28 wherein said control system isimplemented through software on a microprocessor.
 37. The power systemof claim 28 wherein said turbine is connected to said utility grid at adistribution level voltage.
 38. The power system of claim 28 whereinsaid power system is one of a plurality of power systems wherein saidplurality of power systems are connected to said utility grid through acommon substation.
 39. The power system of claim 28 wherein said powersystem is one of a plurality of power systems wherein said plurality ofpower systems are connected to said utility grid at a sub-transmissionlevel voltage.
 40. The power system of claim 28 wherein said powersystem is one of a plurality of power systems wherein said plurality ofpower systems are connected to said utility grid at a transmission levelvoltage.
 41. The fluid flow turbine of claim 28 wherein said controlsystem comprises: a corrective signal of a phase-locked loop; amultiplier connected to said control system input and to said correctivesignal, an output signal of said multiplier being said input linevoltage multiplied by said corrective signal, said output signal furtherbeing a sinusoidal signal component with a DC offset related to a phasedifference between said input signal and said corrective signal; a lowpass filter connected to said output signal of said multiplier, anoutput of said low pass filter being a DC signal absent said sinusoidalsignal component; a proportional-integral (P-I) regulator connected tosaid output of said low pass filter, said P-I regulator having a P-Iregulator output; a summation circuit connected to said P-I regulatoroutput; a triangle wave generator connected to an input of saidsummation circuit, a triangle wave generator output of said trianglewave generator being a steady AC triangle wave, a summation circuitoutput being a sum of said P-I regulator output and said triangle wavegenerator output; and, a cosine function circuit connected to saidsummation circuit output, an output of said cosine function circuitbeing said corrective signal.
 42. The apparatus of claim 41 furthercomprising: a number of parallel phase shift circuits connected to saidoutput of said summation circuit, one such phase shift circuit for eachphase of said multi-phase utility grid; a number of trigonometric sinefunction circuits connected to said a number of parallel phase shiftcircuits, the output of each trigonometric sine function circuit being afixed amplitude reference sinusoidal wave at a frequency appropriate forthe utility grid frequency and substantially in phase with said utilitygrid; a DC value circuit having a DC output which corresponds to adesired AC output current level; and, a number of scaling multipliercircuits connected to said number of trigonometric sine functioncircuits and to said DC value output, outputs of said scaling circuitsbeing scaled sinusoidal signals for each phase.
 43. The apparatus ofclaim 41 wherein said P-I regulator is set to control dynamics of theinverter control circuit response.
 44. The apparatus of claim 42 whereinsaid DC value circuit is set by a turbine controller to set said ACcurrent level proportional to generator torque level.
 45. The apparatusof claim 41 wherein said P-I output signal is scaled such that by addingit to said triangle wave output said corrective signal is phase shiftedto bring it closer to ninety degrees out of phase with said controlsystem input. 46-50. (canceled)
 50. The fluid flow turbine of claim 46wherein said control system comprises: a corrective signal of aphase-locked loop; a multiplier connected to said control system inputand to said corrective signal, an output signal of said multiplier beingsaid input line voltage multiplied by said corrective signal, saidoutput signal further being a sinusoidal signal component with a DCoffset related to a phase difference between said input signal and saidcorrective signal; a low pass filter connected to said output signal ofsaid multiplier, an output of said low pass filter being a DC signalabsent said sinusoidal signal component; a proportional-integral (P-I)regulator connected to said output of said low pass filter, said P-Iregulator having a P-I regulator output; a summation circuit connectedto said P-I regulator output; a triangle wave generator connected to aninput of said summation circuit, a triangle wave generator output ofsaid triangle wave generator being a steady AC triangle wave, asummation circuit output being a sum of said P-I regulator output andsaid triangle wave generator output; and, a cosine function circuitconnected to said summation circuit output, an output of said cosinefunction circuit being said corrective signal.
 51. The apparatus ofclaim 50 further comprising: a number of parallel phase shift circuitsconnected to said output of said summation circuit, one such phase shiftcircuit for each phase of said multi-phase utility grid; a number oftrigonometric sine function circuits connected to said a number ofparallel phase shift circuits, the output of each trigonometric sinefunction circuit being a fixed amplitude reference sinusoidal wave at afrequency appropriate for the utility grid frequency and substantiallyin phase with said utility grid; a DC value circuit having a DC outputwhich corresponds to a desired AC output current level; and, a number ofscaling multiplier circuits connected to said number of trigonometricsine function circuits and to said DC value output, outputs of saidscaling circuits being scaled sinusoidal signals for each phase.
 52. Theapparatus of claim 50 wherein said P-I regulator is set to controldynamics of the inverter control circuit response.
 53. The apparatus ofclaim 51 wherein said DC value circuit is set by a turbine controller toset said AC current level proportional to generator torque level. 54.The apparatus of claim 50 wherein said P-I output signal is scaled suchthat by adding it to said triangle wave output said corrective signal isphase shifted to bring it closer to ninety degrees out of phase withsaid control system input.